1. Field of the Invention
The present invention relates to a nitride semiconductor device employing Group III nitride semiconductors.
2. Description of Related Art
Nitride semiconductor devices such as HEMTs (High Electron Mobility Transistors) and MISFETs (Metal Insulator Semiconductor Field Effect Transistors) which employ Group III nitride semiconductors are known to be used as power devices for power amplifier circuits, motor driving circuits and the like.
Such a nitride semiconductor device is produced, for example, by using an epitaxial wafer including a substrate and an epitaxial layer formed on the substrate by epitaxial growth of a Group III nitride semiconductor. The substrate to be used is preferably of the same type as the epitaxial layer, i.e., a Group III nitride semiconductor substrate, for reduction of a difference in linear expansion coefficient between the substrate and the epitaxial layer.
However, the Group III nitride semiconductor substrate is expensive. In recent years, therefore, a different type of substrate such as a Si substrate or a SiC substrate is used instead of the Group III nitride semiconductor substrate. In this case, a buffer layer is provided between the different type substrate and the epitaxial layer. The buffer layer is made of a composition having a lower average lattice constant than the epitaxial layer, thereby relieving the warpage of the epitaxial wafer which may otherwise occur due to an internal stress caused by a difference in linear expansion coefficient between the different type substrate and the epitaxial layer during cooling after the epitaxial growth.
For example, Patent Document 1 (JP-A-2003-59948) discloses a HEMT which includes a Si substrate, a composite buffer layer including AlxGa1-xN (0<x≦1) sublayers and AlyGa1-yN (0<y≦1) sublayers alternately stacked and covering the entire major surface of the Si substrate, and a HEMT element semiconductor region including a GaN electron transport layer provided on the buffer layer.
The buffer layer is capable of accommodating (or buffering) the difference in linear expansion coefficient between the Si substrate and the GaN electron transport layer, if having a thickness not greater than a certain thickness (critical thickness). If having a thickness greater than the critical thickness, on the other hand, the buffer layer is liable to experience misfit transition to reduce a stress energy. Thus, the buffer layer experiences lattice relaxation and, therefore, has a lattice constant that is closer to its intrinsic lattice constant. As a result, the buffer layer fails to serve for its intended purpose (to provide the buffering effect). Therefore, a great tensile stress acts on the GaN electron transport layer, resulting in cracking of the GaN electron transport layer.
In the prior art, therefore, the thickness of the buffer layer (the number of periods of AlxGa1-xN/AlyGa1-yN of the buffer layer) should be limited. This makes it difficult to improve the breakdown voltage of the device by increasing the thickness of the buffer layer. Such a difficulty is desirably eliminated as far as the properties (e.g., leak current value) of the device are not adversely influenced.